1. The Field of Invention
The invention relates to a process for treading vulcanized tire carcasses. Nowadays, use is made of these processes generally in the context of retreading but, obviously, everything that will be stated hereinbelow is valid for the treading of new tire carcasses.
2. The Related Art
Two main types of retreading processes exist: one, commonly known as “cold retreading”, consisting in using a precured tread that is placed on a carcass ready to be retreaded, and then in curing the assembly using heating means such as an oven; the other process, known as “hot retreading”, consists in placing an uncured tread, in the form especially of sheets, strips or a profile, on a carcass ready to be retreaded, and then in curing the assembly in a mold.
These hot retreading processes apply in particular to the retreading of large-sized tires, such as tires intended for heavy vehicles, agricultural machines, civil engineering vehicles, etc., for which problems of handling and deformation of the treads make it difficult to use them in a precured form. The invention lies in the field of hot retreading.
Among the processes for hot retreading, U.S. Pat. No. 4,053,265 describes a process that consists in using an elastic annular mold, preferably made of rubber, intended for molding and vulcanizing the tread. This elastic mold is placed on the tire carcass to be retreaded, which has been covered beforehand with an uncured tread, the mold-carcass assembly then being entirely enveloped with a membrane that ensures leaktightness. This assembly is then placed in an oven for curing after creating a vacuum in the membrane.
More specifically, in order to be able to ensure perfect leaktightness between the carcass, the tread and the elastic mold, as shown in U.S. Pat. No. 4,053,265 (FIG. 2), it is necessary to envelop the carcass covered with the mold using two elastic membranes: one membrane, known as the “inner membrane”, which extends from the outer surface of the sidewalls over the entire inner surface of the carcass, and a second membrane known as the “outer membrane”, which is covered by the first membrane on the outer surface of the sidewalls, or vice-versa, so as to ensure continuity of the leaktightness between the two membranes. The outer membrane thus covers the entire outer surface of the mold and the portion of the sidewalls not covered by the inner membrane.
These membranes are particularly difficult to install. The reason for this is that it is necessary to stretch the outer membrane very strongly in order for it to be able to cover all of the tread and the sidewalls, this operation being made even more difficult by the fact that the tires involved are generally of a large size. This problem of installation furthermore entails a very high rate of wear of the outer membrane, which is subjected to large elastic stresses and frequent risks of tearing, the consequence of which is that it is necessary to change the membranes frequently, not to mention the risks of using a damaged membrane that would generate defects in the tire produced.
U.S. Pat. No. 4,053,265 proposes an alternative to the use of the two membranes, which consists in using an outer membrane identical to that in the first solution, except that it uses an O-ring seal to achieve the leaktightness at the sidewalls and a membrane inside the tire in the form of an inner tube connected to a support disc of the tire. It is clear that this solution does not solve the abovementioned problems associated with the use of the outer membrane, and that, furthermore, this solution does not make it possible to achieve good leaktightness unless the O-ring seals are vulcanized onto the sidewalls of the tire carcass, which is problematic.
Applicant's commonly-owned U.S. patent application Ser. No. 09/956,154, filed on Sep. 19, 2001 and published on Apr. 11, 2002 as US-2002-0040755-A1 (and issued as U.S. Pat. No. 6,824,633 B2), describes a device for treading a vulcanized tire carcass using an uncured tread comprising a mounting rim and an elastic annular mold for molding the tread. This device also comprises two elastic flanges intended to cover, respectively, the outer surface of the tire thus made, between a bead of the tire and the peripheral edge of the corresponding mold, each flange being extended on its larger-diameter peripheral end with an elastic sleeve covering the shoulders and partially the crown of the elastic mold.
Such a device makes it possible to overcome the drawbacks mentioned above, but the problem of the longevity of such flanges remains. The reason for this is that the flanges used in this device must at the same time ensure very good leaktightness and have great elasticity. These properties are particularly demanded during the curing stage where, since the elastic mold enters the rubber tread mix, its diameter decreases and the flanges, or more specifically their extensions in the form of sleeves covering the crown of the mold, must follow these variations in diameter while continuing to ensure leaktightness. Rubbers that allow this compromise of properties do not allow the flanges to be used for more than about ten cycles.
The present invention is directed towards overcoming all of the abovementioned drawbacks.